1. Field of the Invention
The present invention relates to an atomic force microscope (AFM) adapted to evaluate physical properties of a sample. The present invention also relates to a method of energy dissipation imaging using the AFM.
2. Description of Related Art
Ultrasonic atomic force microscopy (UAFM) has been developed as a technique for evaluating the contact resilience of the sample surface portion in contact with the probe of a cantilever from the resonance frequency of a flexural mode of the cantilever of an atomic force microscope (AFM) operating in contact mode. A sample-evaluating method using the UAFM has the feature that the contact resilience of a hard sample can be evaluated using a softer cantilever than the cantilever used in the contact resilience-evaluating technique using the force modulation mode. Therefore, the evaluation method using the UAFM is adapted for evaluation of metals, ceramics, and semiconductors (see, for example, K. Yamanaka and S. Nakano, Jpn. J. Appl. Phys. 35.93 (1996).
The energy dissipation characteristics of the portion of the sample surface which is in contact with the cantilever probe can be evaluated approximately from the Q factor defined as the ratio of the resonance peak width to the resonance frequency (see, for example, O. Wright and N. Nishiguchi, Appl. Phys. Lett. 71, 626 (1997).
Another method for evaluating the resilience characteristics more completely is also proposed, for example, in K. Yamanaka and S. Nakano, Appl. Phys. A, 66, S313 (1998). In this method, the Young's modulus, shear modulus, and Poisson's ratio are separated, using a torsional mode of a cantilever.
A further method of imaging the energy dissipation in the portion of a sample surface in contact with the cantilever probe is proposed. In this method, the imaging is performed by increasing the speed of scanning of the cantilever and mapping the Q factor at each pixel.
This ultrasonic atomic force microscope is similar to the non-contact atomic force microscopy (NC-AFM) using a frequency modulation mode, in that the resonance of the cantilever is used. However, there exists a fundamental difference. That is, in NC-AFM, the cantilever vibrates at large amplitudes exceeding 10 nm, so that the cantilever probe moves away from the sample. Meanwhile, in ultrasonic AFM, the cantilever vibrates at small amplitudes of less than 1 nm while the probe is kept in contact with the sample.
As a result, the force gradient that is the ratio of displacement to force or the contact resilience remains almost constant over one whole cycle of vibration. This produces the advantage that it is possible to realize accurate quantitative evaluation. Therefore, this technique is anticipated as a novel method of lattice defect analysis that compensates for the drawbacks with the techniques for evaluating electronic and mechanical materials in the fields of nanotechnology and electron microscopy.
The configuration of a conventional atomic force microscope is shown in FIG. 2, where a sample 21 is placed on a sample stage 4. A probe 20 is mounted at the tip of a cantilever 1 and located opposite to the sample 21.
Light from a laser diode (LD) 2 is directed at the cantilever 1. The reflected light is detected by a split photodiode (FD) 3 via a mirror 24. The output signal from the photodiode 3 is sent to a calculation portion 5, which produces a cantilever signal indicative of the flexure of the cantilever 1.
The cantilever signal from the calculation portion 5 is split into two parts. One of the parts is input into a z-motion controller 7 via a low-pass filter (LPF) 6. The z-motion controller 7 controls the z-position of the sample 21 placed on the sample stage 4.
The other part of the cantilever signal is input to a phase comparator 12 via a band-pass filter (BPF) 11. Furthermore, the output signal from an oscillator 8 is amplified by an amplifier 9 and then split into two parts and applied to the phase comparator 12 via a variable phase shifter 22. The output signal Vp from the phase comparator 12 is applied to an error amplifier 13. Furthermore, a reference voltage signal Vref is also applied to the error amplifier 13, which in turn produces an output signal VE proportional to the difference between the two signals, or error. The output signal VE is applied to an adder 16 via a switch 14.
The output Vo from a voltage source circuit 15 is also applied to the adder 16. The output V from the adder 16 is input into the oscillator 8. The output from the oscillator 8 is fed back to an ultrasonic oscillator 10 via the amplifier 9.
The output signal from the band-pass filter 11 is fed to an amplitude detector 17 and to a frequency demodulator 18. The output signals from the amplitude detector 17 and frequency demodulator 18 are applied to an imaging device 19. The imaging device 19 creates a Q factor image based on the output signal from the amplitude detector 17 and creates a resonant frequency image based on the output signal from the frequency demodulator 18.
To obtain an energy dissipation image, it is intrinsically necessary to visualize the excitation energy applied from the outside such that the amplitude of the cantilever is kept constant.
In the conventional technique, however, the cantilever is oscillated at a constant excitation voltage. The output obtained at this time from the amplitude detector 17 is directly fed to the imaging device 19. The imaging device 19 approximately creates an energy dissipation image directly based on the output from the amplitude detector 17. See K. Yamanaka et al., Appl. Phys. Lett. 78, 1939 (2001); Japanese Patent Laid-Open No. 2002-277378; and U.S. Pat. No. 6,983,644.
In the conventional technique, in a case where an energy dissipation image is obtained, the cantilever is vibrated at a constant excitation voltage. An energy dissipation image has been approximately created based on the output itself obtained at this time from the amplitude detector. In this conventional technique, it has been impossible to obtain an accurate energy dissipation image.